1. Field of the Invention
This invention relates to a temperature fuse and an apparatus for detecting abnormality of a wire harness for a vehicle, and the invention is best suited for detecting an abnormal heat generation caused in the wire harness due to, for example, a rare short (short-circuit).
The present application is based on Japanese Patent Application No. Hei. 9-179536, which is incorporated herein by reference.
2. Description of the Related Art
In order that lead wires can be positively insulated from each other when a fuse element is melted, there have heretofore been extensively used, as temperature fuses (heat-sensitive fuses), surface tension-operating type fuses in which a fuse element 1 is made of a material liable to adhere to lead wires 2 when the fuse element 1 is melted, as shown in FIGS. 5 and 6.
This fuse element 1 is formed by adding an appropriate amount of lead to a binary eutectic alloy of 47-49 wt. % tin and 53-51 wt. % indium so that the fuse element 1 can be melted in a temperature range of 120 to 135.degree. C., and in this temperature fuse, the molten fuse element 1 provides a sufficient surface tension, and a variation in operating temperature is much reduced. This temperature fuse is proposed in Unexamined Japanese Patent Publication No. Sho. 56-114238.
Such a temperature fuse has been extensively used as a temperature detection portion in various electronic equipments. Here, let's consider the case where such a temperature fuse is used as means for detecting the temperature of a wire harness installed in a vehicle.
First, the need for detecting the temperature of a wire harness will be described. In a vehicle, generally, electric power, produced by a power source such as a battery and an alternator, is supplied to various loads through a wire harness comprising a plurality of wires. As shown in FIG. 7, this wire harness 11 comprises many conductors 11a covered and protected by an insulating sheath 11b made, for example, of polyvinyl chloride. Namely, the wire harness is bundled and protected by fitting soft vinyl chloride thereon or by winding a tape therearound, thereby preventing the (electricity) leakage from the conductors 11a.
Generally, a wire harness is connected to a fuse (which is a current-interrupting fuse, and is different from a temperature fuse), and this arrangement prevents damage to loads and the deterioration of wires which would be caused by an excessive current produced when a voltage variation develops in a power source (such as a battery and an alternator) and when a dead short of the wire develops.
The connection of this wire harness to the fuse is made, for example, by a fuse box 12 shown in FIG. 8. The fuse box 12 comprises a housing 15 of an insulative, heat-resistant resin in which a fuse element 13 is received, and one end of the wire harness is connected to terminals 14a and 14b provided respectively at opposite ends of the fuse element 13.
Generally, a fuse, used in a vehicle, has such melting characteristics that the fuse is melted immediately when a current of a value more than 200% of its rated value flows therethrough, but the melting time for a current of a value less than 200% of the rated value is relatively long since the fuse is designed to withstand a rush current.
There are occasions when the conductors 11a are brought into contact with a vehicle body as a result of aged deterioration of the insulating sheath 11b or wear of the insulating sheath 11b by contact with an edge portion of the vehicle, that is, a so-called dead short or a so-called rare short (intermittent short-circuit) develops. In the former case, a large current flows through the fuse, and therefore the fuse is melted immediately, but in the latter case, the heat generation and heat radiation alternately occur in the fuse, so that there is a tendency; that a longer time is taken until the fuse is melted.
Therefore, when the rare short continues, there is a possibility that the insulating sheath 11b of the wire harness 11 serves as a heat accumulator before the melting of the fuse, so that the temperature of the insulating sheath 11b increases. As a result, the insulating sheath 11b is further deteriorated.
Therefore, it is thought that if there is a temperature fuse capable of detecting the temperature of the wire harness, this is very convenient. It is particularly desired to provide a temperature fuse capable of accurately detecting a temperature condition in which the wire harness rises to a temperature in the range of between 140.degree. C. and 160.degree. C. Referring to the reason for this, in view of the material of the wire harness, if the temperature lower than 140.degree. C. is detected, and this is judged as an abnormal condition, the wire harness is decided as being abnormal even though the wire harness is not still much deteriorated, and therefore this not suitable. In contrast, if the relevant temperature is more than 160.degree. C., there is a possibility that the deterioration of the wire harness is not prevented. Therefore, it is necessary to provide the temperature fuse which is melted accurately at the desired predetermined temperature in the range of 140.degree. C. and 160.degree. C.